Stable lead alkyl compositions and a method for preparing the same



United States Patent M 3,021,350 STABLE LEAD ALKYL COMPOSITIONS AND A METHOD FOR PREPARING THE SAlVlE Shlr] E. Cook and Hymin Shapiro, Baton Rouge, La.,

assignors to Ethyl Corporation, New York, N.Y., a

corporation of Delaware N0 Drawing. Filed Nov. 21, 1960, Ser. No. 70,410

4 Claims. (Cl. 260-437) This invention relates to alkyllead compositions which are stable at temperatures above 100 C. It also relates to methods for inhibiting the thermal decomposition of alkyllead compounds when subjected to temperatures above 100 C., at which temperature thermal decomposition becomes appreciable.

Generally this invention contemplates inhibiting the thermal decomposition of alkyllead compounds in which at least one valence of the lead is satisfied by an alkyl radical.

More specifically, this invention is concerned with an improved process for separating alkyllead compounds from the reaction products accompanying their synthesis. It is also applicable to a method for inhibiting thermal decomposition of an alkyllead product during its purification and blending with other products in making commercial antiltnock fluids. It is applicable to minimizing the possibility of thermal decomposition during storage or transportation of an alkyllead product. It is especially applicable to preventing thermal decomposition of undiluted alkyllead compounds where the likelihood of thermal decomposition is more of a problem.

As is well known, tetraalkyllead antiknock compounds generally are produced by reacting a sodium-lead alloy with an alkyl halide. Due to recent marked improvements in the technology of alkyllead manufacture, thermal instability of alkyllead compounds during synthesis is no longer a problem. However, the tetraalkyllead compound so produced is in admixture with various reaction byproducts from which it must be separated. Separation is effected by steam or vacuum distillation with subsequent purification of the tetraalkyllead distillate. Due to the toxic and unstable nature of tetraalkyllead antiknock compounds, these distillation and purification operations are subject to many difficulties.

*In these distillation and purification operations meticulous temperature control and exact safety measures are of paramount importance. The rate of decomposition of the alkyllead compound increases rapidly with small rises in temperature above the temperature where thermal decomposition becomes appreciable. For example, decomposition of tetraethyllead occurs at the rate of approximately 2 percent per hour at a temperature of 100 C., which is the customary temperature used in separating tetraethyllead from the reaction products accompanying its synthesis. At temperatures above 100 C., the decomposition rate increases logarithmically so that a point is soon reached where external heat is no longer required and decomposition becomes selfpropagating.

Such likelihood of excessive decomposition is present also during blending, handling, storage, and transportation. Prior to diluting the alkyllead concentrate with scavengers, gasoline or other materials, the alkyllead compound remains a concentrate and the problem of excessive thermal decomposition exists, even though the temperature is maintained normally well below that of decomposition. For example, in the purification step wherein the tetraethyllead concentrate is washed and blown with air at atmospheric temperature to remove impurities, a sudden increase in temperature may occur due to the oxidation of triethylbismuth which is present as an impurity. Also pumps used in handling tetraethyllead occasionally freeze and the friction developed may cause a local overheating to a temperature above the temperature of 3,021,350 Patented Feb. 13, 1962 decomposition of the tetraethyllead. Faulty wiring,.leaks onto steam pipes, and other accidental causes also may produce local overheating with resulting dangerous decomposition.

It is seen therefore that in those operations where an alkyllead compound is in the undiluted or concentrated state-viz separation, purification, blending, transporta tion, and storage-the likelihood of excessive thermal decomposition must be provided for and effectively combatted.

An object of this invention is to stabilize alkyllead compounds against thermal decomposition during one or more of the following operations: separation, purification, blending, transportation and storage.

The above and other objects of this invention are accomplished by incorporating with alkyllead compounds a relatively small quantity of a mixture of materials which has the property of synergistically inhibiting alkyllead thermal decomposition. The foregoing objects are also accomplished by conducting one or more of the foregoing operations in the presence of such a mixture of materials. The mixtures which have been found to possess these unexpected properties are referred to hereinafter as the.- mal stabilizers.

The synergistic thermal stabilizer mixtures of this invention are composed of ethylene dibromide and trimethyl phosphate. The concentrations in which they are used are equivalent to from about 0.25 to about 0.50 mole of ethylene dibromide per mole of alkyllead compound, and from about 0.1 to about 0.4 mole of trimethyl phosphate per mole of alkyllead compound. These thermal stabilizers when used in these amounts are very effective in substantially retarding or preventing thermal decomposition of the alkyllead compound at temperatures ranging from about C. up to about 195 C, for extended periods of time. additives in this regard is synergistic, i.e. the thermal stabilization efiectiveness of the whole is far greater than the sum total of its parts.

The chief thermal decomposition products of alkyllead compounds are lead metal and hydrocarbon gas. Hence, a very good index of alkyllead thermal decomposition is liberation of this gas.

To illustrate the effectiveness of this invention, a series of direct comparisons were made of the decomposition characteristics of unsta'oilized and stabilized tetraethyllead samples. A thermostatically controlled hot oil bath was fitted with a stirrer, thermometer, and aholder for a small reaction tube. A 100 cc. gas buret beside the path, and equipped with a water-containing levelling bottle, was connected by means of rubber tubing with the reaction tube after the desired sample was introduced into this tube. After the bath was brought to a steady temperature of 195 C., the sample-containing tube was quickly immersed in the bath and clamped with the levelling bottle adjusted to hold the gas buret in place at a zero reading. Then measured was the time during which the sample was held at 195 C. without pronounced thermal decomposition and consequent gas evolution occurring. Thus, the longer the time, the more thermally stable was the alkyllead composition.

With pure tetraethyllead used in 1 ml. amounts, pronounced thermal deterioration occurred almost immediately as evidenced by rapid gas evolution. In fact, the decomposition of unstabilized tetraethyllead will normally become uncontrollable if it is heated, whether rapidly or slowly, to even C., unless it is possible to very rapidly cool it down to about 100 C. or below.

The remainder of the compositions tested in the manner described above and the results thereby obtained are shown in the following table.

Moreover, the behavior of this combination of 3 v TABLE It will be noted that the compositions of this invention exhibited a high degree of synergistic effectiveness.

The above-described beneficial behavior of the thermal stabilizer mixtures of this invention also takes place with other alkyllead compounds such as triethyllead bromide and tetrapropyllead. In fact, these compounds when stabilized can be boiled and distilled at atmospheric pressure.

This invention is adapted to 'the stabilization of tetra ethyllead and other alkyllead compounds at various stages after they have been formed and the diluents or excess alkyl halide have been discharged from the autoclave. For example, one of the above thermal stabilizer combinations may be added in appropriate quantity to the alkyllead reaction concentrate just before the separation step which is conducted at a temperature close to the temperature where hazardous run-away decomposition is particularly prevalent. By adding one of the above thermal stabilizer combinations to the reaction concentrate/just prior to distillation, the danger arising from unexpected temperature increases is substantially eliminated.

Most preferably the above thermal stabilizer combinations are employed to stabilize the alkyllead compound both in storage and in shipping and especially to stabilize any alkyllead concentrate, i.e., compositions containing at least 80 percent by weight of alkyllead compound. If elevated .temperature conditions are likely to be encountered, the addition of a small amount of thermal stabilizer mixture to the 'alkyllead compound will economically and satisfactorily eliminate most of the hazard involved. While meticulous temperature control and exacting safety measures have been successful in reducing to a minimum the hazards of processing and handling of tetraethyllead, the use of this invention provides a much greater factor of safety. Furthermore, waste of the alkyllead product due to decomposition is considerably minimized through the use of this invention.

This invention is useful in stabilizingalkyllead com-- pounds in which at least one valence of the lead is satisfied by an alkyl radical. For example tetraethyllead, tetramethyllead, tetrapropyllead, dimethyldiethyllead, triethylphenyllead, and triethyllead bromide can be successfully stabilized against thermal decomposition by incorporating therein a relatively small quantity of one of the thermal stabilizers of this invention. This invention is particularly well suited to the stabilization of any mixture involving two ormore of the following compounds: tetramethyllead, ethyltrimethyllead, diethyldimethyllead, triethylmethyllead, and tetraethyllead.

What is claimed is:

1. A method of inhibiting the decomposition of an alkyllead compound at temperatures of from about C. to about C. which comprises incorporating with said compound from about 0.25 to about 0.50 mole of ethylene dibromide per mole of said compounrhand from about 0.1 to about 0.4 mole of trimethyl phosphate per mole of said compound.

, 2. In the process of producing an alkyllead compound by reacting a sodium lead alloy with alkyl chloride and separating the thus produced alkyllead compound from the reaction mass by steam distillation, the step which comprises conducting said steam distillation in the presence of from about 0.25 to about 0.50 mole of ethylene dibromide per mole of said compound, and from about 0.1 to about 0.4 mole of trimethyl phosphate per mole of said compound.

3. An alkyllead compound containing, as a synergistic thermal stabilizer at temperatures of from about 100 C. to about 195 C., from about 0.25 to about 0.50 mole of ethylene dibromide per mole of said compound, and from about 0.1 to about 0.4 mole of trimethyl phosphate per mole of said compound.

4. The composition .of claim 3 wherein saidcompound is selected from the group consisting of tetramethyllead,

. triethylmethyllead, tetraethyllead, and mixtures thereof.

References Cited in the file of this patent UNITED STATES PATENTS 2,660,594 Calingaert Nov. 24, 1953 

1. A METHOD OF INHIBITING THE DECOMPOSITION OF AN ALKYLLEAD COMPOUND AT TEMPERATURES OF FROM ABOUT 100* C. TO ABOUT 195* C. WHICH COMPRISES INCORPORATING WITH SAID COMPOUND FROM ABOUT 0.25 TO ABOUT 0.50 MOLE OF ETHYLENE DIBROMIDE PER MOLE OF SAID COMPOUND, AND FROM ABOUT 0.1 TO ABOUT 0.4 MOLE OF TRIMETHYL PHOSPHATE PER MOLE OF SAID COMPOUND. 